This application claims the priority of 196 28 796.0, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an exhaust emission control system with at least two nitrogen oxide adsorber parts alternately operated in adsorption and desorption modes, and apparatus for conducting further downstream, the exhaust stream emerging from the adsorber part that is currently operating in the adsorption mode. The adsorber part currently operated in the desorption mode recycles the emerging exhaust gas stream into an intake line of the engine.
A known emission control system is described in DE 43 19 294 C1. In one of the systems shown therein, the exhaust line runs from the exhaust outlet of the engine to a control valve that conducts the exhaust stream alternately into one of two exhaust line branches connected downstream. Each of the branches contains an adsorber for nitrogen oxides. Alternative systems are proposed containing only one adsorber for nitrogen oxides and a three-way catalytic converter. In a first such alternative version, the catalytic converter and adsorber are located in two parallel exhaust lines, and a cross line containing an associated control valve is provided between the two branches to connect the catalytic converter output with the adsorber input during adsorption operation. During desorption mode, the exhaust is scrubbed only by the catalytic converter. In a second alternative version the catalytic converter is located downstream from a connector where an exhaust line branch containing the adsorber for nitrogen oxides merges with a bypass line branch. To reinforce the desorption process, provision of a hot gas or inclusion of an electrically operated heating device is proposed.
An object of the present invention is to provide an exhaust emission control system with an exhaust scrubbing function that is further improved over the above-described known systems.
In an exhaust emission control system disclosed in JP 5-195756 (A) for an internal combustion engine, an oxidizing converter located near the engine oxidizes nitrogen oxides. In the flow direction of the exhaust downstream from the nitrogen oxide oxidizing converter, a jet injects a supplied reducing agent into the exhaust stream. Downstream from the jet there are then provided, in sequence, a nitrogen oxide converter and another oxidizing converter that oxidizes the hydrocarbons and carbon monoxide contained in the exhaust.
This object has been achieved by an exhaust emission control system in which oxidizing converter located near the engine and upstream from adsorber parts is provided for oxidation of at least the NO contained in the exhaust to NO.sub.2 and an exhaust line section located upstream of adsorber parts is divided into a main line branch and a partial line branch that is parallel to the main line branch, shorter than the latter, and/or thermally insulated to a greater degree.
The two adsorber parts are connected by control valves to the main line branch and the partial line branch in such fashion that the adsorber part that is operating in the adsorption mode is fed by the exhaust stream from the main line branch and the other adsorber part that is operating in the desorption mode is supplied by the exhaust stream from the partial line branch or an oxidizing converter located near the engine and upstream from adsorber parts is provided for oxidation of at least the NO contained in the exhaust to NO.sub.2.
The two adsorber parts are formed as integral parts of a cylindrical adsorber body through which the flow passes in the lengthwise direction, said body being divided into the two adsorber parts by at least one sector-dividing diaphragm located on the inlet side, with a relative rotary movement being provided around a rotational axis parallel to the flow direction between the sector-dividing diaphragm, provided at least on the inlet side, and the adsorber body.
In this new and advantageous system, an oxidizing converter is located downstream from the parts that adsorb nitrogen oxides near the engine, with the catalytic converter oxidizing the exhaust component NO to NO.sub.2 comparatively soon following an engine start. Because many adsorber materials adsorb NO.sub.2 much better than they absorb NO, higher NO.sub.x absorption rates can be achieved by adsorber parts connected downstream that adsorb nitrogen oxides. Also, the oxidizing converter can be used to oxidize other exhaust components such as carbon monoxide and hydrocarbons. Locating the adsorber parts that. adsorb nitrogen oxides as far away from the engine as possible is advantageous for achieving a low temperature level for NO.sub.x adsorption so that adsorption materials can be used that are not resistant to high exhaust temperatures, and hence to high desorption temperatures as well, or which have little or no ability to adsorb NO.sub.x at high temperatures.
In the exhaust emission control system according to one advantageous embodiment of the present invention, an exhaust line section located upstream from the adsorber parts is divided advantageously into a main flow line branch and a partial flow line branch. As a result of the shorter length and/or better thermal insulation of the partial flow line branch, its exhaust temperature at the outlet, and hence the temperature of the exhaust partial stream fed from this branch into the respective adsorber parts, is clearly above the corresponding temperature of the main exhaust stream guided through the main flow branch. Also, the colder exhaust main stream is fed to the respective adsorber part in the adsorption mode and the warmer exhaust partial stream is supplied during the desorption mode. Consequently, in the desorption mode the adsorber part is heated by the partial stream of exhaust so that the nitrogen oxides stored in it are desorbed and recycled into the intake line of the engine. Separate heating of the adsorber parts in the desorption mode is therefore not necessary at all or only to a slight degree, thus improving the energy balance of the system. As a result of the use of the partial exhaust stream, no additional stream of purge air is required for the desorption mode of the adsorber parts.
The oxidizing converter is favorably positioned inside an exhaust line section that is common to main line branch and partial line branch and is located upstream of both of them, or is located within an inlet area of main line branch. When the oxidizing converter is located within the main flow line branch, the hydrocarbons in the exhaust partial stream used for desorption of the nitrogen oxides the respective adsorber part are not oxidized. This can be utilized in the case of adsorption materials in which competing adsorption takes place between the hydrocarbons and the nitrogen oxides. The nitrogen oxides stored in the respective adsorber part in this case are more easily desorbed in the desorption mode because of the competing adsorption.
As an alternative configuring the two adsorber parts as separate absorber bodies, the other embodiment of the exhaust emission control system integrally combines them within a common absorber body. The body is in turn divided into the two adsorber parts by a suitable sector-dividing diaphragm located on the inlet side and preferably by a corresponding diaphragm on the outlet side. A relative rotary movement between the sector-dividing diaphragm provided at least on the inlet side and the adsorber body, with a rotational axis running parallel to the exhaust flow direction, allows each partial volume of the adsorber body to be in the sector range for the adsorption mode for an adjustable period of time and then in the sector area for the desorption mode.
According to another aspect of the present invention, desorption is supported by adding a desorption additive, so that effective desorption can take place even at comparatively low temperatures. As a desorption-promoting component, this additive contains at least hydrocarbons and possibly other components that can be obtained when used in a motor vehicle for example, from the fuel provided for the engine.
In a further system improvement according to the present invention, at least a part of the heat energy is supplied to the adsorber part operating in desorption mode by burning the fuel provided for the engine. The fuel is carried along in any event for use in, for example, a motor vehicle. In this connection, the burner is advantageously operated so that the hot gas produced has a desorption-promoting composition, i.e. it contains little or no oxygen, but unburned hydrocarbons.